Mosquitoes are blood-sucking arthropods of major public health significance, serving as vectors of several arboviruses, malarial parasites, and filarial worms. Immature mosquitoes are aquatic, micro-plankton-feeding insects while the adults are terrestrial-aerial and blood and nectar feeders. The profound transition (metamorphosis) from the immature to adult form occurs in the so-called pupal stage. Compared to the egg, larval and adult stages, the pupal stage has received little attention. Mosquito pupae are aquatic and are buoyed up in the water by a bubble of gas in a cavity (the ventral air space) formed by their developing mouthparts, antennae, legs and wings. During the past year I have found that the volume of this gas is variable and appears to be regulated. As a result, buoyancy appears to be regulated. I am proposing to investigate the factors which are involved in this variation in volume and buoyancy and the mechanism by which volume is determined. The methods to be applied in this study include the following: (1) standard histological techniques; (2) light microscopy; (3) scanning electron microscopy; (4) microsurgical techniques; and (5) several special techniques which I have devised in order to study pupal buoyancy.